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Abstract

We present a femtosecond frequency comb vernier spectrometer in the near infrared with a femtosecond Er doped fiber laser, a scanning high-finesse cavity and an InGaAs camera. By utilizing the properties of a frequency comb and a scanning high-finesse cavity such a spectrometer provides broad spectral bandwidth, high spectral resolution, and high detection sensitivity on a short time scale. We achieved an absorption sensitivity of ~8 × 10−8 cm−1Hz-1/2, corresponding to a detection limit of ~70 ppbv for acetylene, with a resolution of ~1.1 GHz in single images taken in 0.5 seconds and covering a frequency range of ~5 THz. Such measurements have broad applications for sensing greenhouse gases in this fingerprint near infrared region with a simple apparatus.

Vernier reference and sample images: (a) only air, (b) 5 ppmv acetylene,
vernier ratio ~250/249; (c) only air, (d) 5 ppmv acetylene, vernier
ratio ~500/499. The varying brightness along the x axis
is caused by the non-uniform spectral density distribution of the near
infrared frequency comb source; for the y axis,
especially at the top of images (beginning of the scan), it is caused by
the nonlinearity of the PZT and Galvo scans. Along the
y axis, the acetylene absorption features are quite
clear for both vernier ratios.

Image processing procedure for generating an absorption spectrum from a
pair of reference and sample images: a pair of reference and sample
images (1) is used to generate an absorption image (2); after
determining the vertical boundaries for a unique data set (two black
lines across the absorption image), the fringes between the boundaries
are unwrapped into a traditional one dimensional absorption spectrum
(3).

Acetylene absorption spectrum retrieved from images of ~500/499 and
~250/249 vernier ratio (black) and comparison with HITRAN simulation of
5 ppmv acetylene at room temperature and atmospheric pressure (red,
inverted for clarity): (a) in the broad range between 1510~1550 nm and
(b) expanded view between 1520~1530 nm for a vernier ratio ~500/499; (c)
in the broad range between 1510~1550 nm for a vernier ratio ~250/249.
Due to the reduced power of the resolved frequency elements the
signal-to-noise ratio in this case is lower compared to the case of
~500/499 vernier ratio.